1. Field of the Invention
The present invention relates to a formulation for depositing a material on a substrate, to a method of depositing a material on a substrate particularly by an ink-jet deposition technique, and to a method of producing a light-emitting device by such a deposition method.
2. Related Technology
As shown schematically in FIG. 1, a light-emitting device typically comprises a layer of an electroluminescent polymer 3 sandwiched between a cathode 2 and an anode 1 such that charge carriers can move between the electrodes and the layer of electroluminescent polymer. It is typically produced by depositing a layer of electroluminescent polymer 3 on a glass substrate 5 coated with an anode layer 1 such as a layer of indium-tin oxide (ITO) and depositing a cathode layer 2 such as a calcium layer over the layer of electroluminescent polymer 3. The device may comprise further layers, such as a hole transport layer 4 (such as a layer of doped polyethylene dioxythiophene as described in EP0686662) provided between the anode 1 and the electroluminescent polymer layer 3 and an electron transport layer provided between the cathode and the electroluminescent layer (not provided in the device shown in FIG. 1).
An ink-jet technique may be used for depositing the layer of electroluminescent polymer. Such a technique is described in EP0880303A1, whose content is incorporated herein by reference. This technique basically involves the controlled deposition of drops of a solution of the electroluminescent polymer through a nozzle followed by evaporation of the solvent. This technique is particularly suited to the deposition of patterned layers of the electroluminescent polymer. For example, in some applications, it may be required to have a layer of electroluminescent polymer comprising an ordered array of pixels, wherein each pixel is produced by the deposition of a single drop of solution of the electroluminescent polymer. It is desirable in such cases that the polymer is distributed uniformly in the spot remaining after evaporation of the solvent.
Solutions of the electroluminescent polymer in solvents such as isodurene have conventionally been used in this technique. However, there has been noticed the problem with conventional solvents that after a drop of the deposited solution has dried most of the electroluminescent polymer is deposited as a ring around the edge of the remaining spot leaving only a very thin film of the polymer at the centre of the spot. This can result in relatively poor device efficiency.
It is an aim of the present invention to provide a formulation with which a film of the desired thickness profile can be deposited on a substrate according to a technique involving controlled drop deposition.